BTW, the "KF8 Example" provided by Amazon web site in the "Kindle Format 8" page has the same problem. I informed Amazon support 1 week ago so they have a test case to investigate and got no answer so far.

This looks suspiciously like a problem I'm seeing. On a first-generation Kindle Fire, the book opens up fine to Chapter 1, but on a newer Fire, I'm getting reports that it opens up in the ToC. It's one single file, and I'm pointing to the beginning like so:

Updating to 2.8 has no effect. What I find extremely strange about the situation is that things work fine on my Kindle Fire, but on a Fire HD, the book apparently opens up in the middle of the ToC. I didn't think the two systems were very far out of sync in terms of things like that. I had hoped not to have to redo everything by breaking out every chunk of text into its own HTML file.